Many vehicles include pneumatic systems that require compressed air to function. For example, trucks, buses, trailers, and trains use air brakes in which compressed air is supplied to a brake chamber to provide the force required to actuate the brakes. Air suspensions, which use compressed air as a spring to replace the steel springs of a conventional suspension system, are another type of pneumatic system included in some vehicles. Many heavy duty trucks also include air seats and air horns, both of which require a source of compressed air.
Safe and reliable operation of these and other pneumatic systems requires that an adequate supply of compressed air be readily available for use by the systems. To ensure that compressed air is available, atmospheric air is compressed by a compressor and stored in one or more compressed air storage tanks until such air is needed. This supply of compressed air is depleted as compressed air is supplied to the pneumatic systems. A certain amount of the compressed air supply is also lost due to leaks in the pneumatic systems.
The amount of compressed air stored in a storage tank is controlled by regulating the amount of compressed air supplied to the storage tank by the compressor. Typically, a governing mechanism controls the flow of compressed air into the storage tank so that the pressure inside the storage tank is maintained between predetermined maximum and minimum pressures. The maximum pressure is determined based on design considerations involving at least the capacity of the compressor, the governing mechanism, the storage tank, and the pneumatic systems to which the compressed air is supplied. The minimum pressure is generally the pressure required to assure that the pneumatic systems remain operable.
Referring now to FIG. 1, a currently known compressed air charging system 10 includes a compressor 12, a storage tank 14, and a mechanically controlled governor 16. The governor 16 detects the pressure in the storage tank 14 and selectively directs compressed air from the compressor 12 to storage tank 14 based on the storage tank pressure. When the pressure in the storage tank 14 drops below a predetermined lower limit, e.g., 100 psi, a valve in the governor 16 directs the air from the compressor 12 into the storage tank 14 to charge the storage tank 14. When the pressure in the storage tank 14 reaches an upper limit, e.g., 120 psi, the storage tank 14 is fully charged, and the valve in the governor 16 directs air from the compressor 12 through a discharge outlet 18 to the atmosphere.
Compressors of these systems 10 are generally driven by the vehicle engine. When the tank is not being charged, air from the compressor is discharged to the atmosphere, and very little engine power is required to drive the compressor. However, the same compressor may require approximately 4 to 5 horsepower when air from the air compressor is used to charge the storage tank. As a result, charging the storage tank uses engine power that would otherwise be available to the vehicle operator.
Under certain conditions, a driver may require all available engine power. For example, when a truck travels up a grade, the truck generally demands all available power. If the compressor is charging the tank during this time, not all of the engine power is available to power the truck up the grade. Further, because the driver is unlikely to use the brakes while traveling up a grade, it is reasonable to use all available power to move the truck and leave charging the storage tank until a later time, as long as enough pressure is maintained in the storage tank to operate other systems in a safe manner.